1. Field of the Invention
This invention relates to a rear wheel turning system for a four-wheel steered vehicle, and more particularly to a rear-wheel turning system in which the rear wheel turning angle relative to the front wheel turning angle is changed according to the yaw rate of the vehicle.
2. Description of the Prior Art
In a rear wheel turning system disclosed in Japanese Unexamined Patent Publication No. 57(1982)-44568, the rear wheels are turned according to the lateral acceleration or the yaw rate of the vehicle in order to compensate for influence of external disturbance such as side wind. That is, the rear wheel turning system comprises a rear wheel turning mechanism which turns the rear wheels, a yaw rate detecting means which detects the yaw rate of the vehicle, and a control means which causes the rear wheel turning mechanism to turn the rear wheels by a target rear wheel turning angle which is determined according to the yaw rate as detected by the yaw rate detecting means.
By such a rear wheel turning system, the running stability of the vehicle can be improved. However since the rear wheels are turned in the same direction as the front wheels when the front wheels are turned, the heading performance of the vehicle deteriorates. In this specification, that the rear wheels are turned in the same direction as the front wheels will be expressed as "the rear wheels are turned in the same phase", and that the rear wheels ar turned in the direction opposite to the front wheels is expressed as "the rear wheels are turned in the reverse phase".
When the rear wheels are momentarily turned in the reverse phase at the beginning of a turn, desired heading performance can be obtained and at the same time the subsequent running stability of the vehicle can be ensured. More particularly, when the target rear wheel turning angle is defined by the following formula, the rear wheels are momentarily turned in the reverse phase from the time the front wheels are turned to the time the position of the vehicle is actually changed and the yaw rate is detected. EQU TG.theta..sub.R '=-K.sub.1 .multidot..theta..sub.F +K.sub.2 .multidot.V.multidot..PSI. (1)
wherein TG.theta..sub.R ' represents the target rear wheel turning angle, K.sub.1 and K.sub.2 respectively represent positive coefficients, .theta..sub.F represents the front wheel turning angle, V represents the vehicle speed and .PSI. represents the yaw rate. When the rear wheel turning angle is positive, the rear wheels are turned in the same phase and the rear wheel turning angle is negative, the rear wheels are turned in the reverse phase.
However, when the target rear wheel turning angle is determined according to the formula (1), the control system can oscillate because the signal which represents the actual position of the mechanical system and is fed back to the control system is apt to lag behind the control signal.
When the yaw acceleration .PSI. is added to the formula (1) as the following formula (2) and the target rear wheel turning angle TG.theta..sub.R defined by the formula (2) is used, such an oscillation of the control system can be suppressed. EQU TG.theta..sub.R =-K.sub.1 .multidot..theta..sub.F +K.sub.2 .multidot.V.multidot..+K.sub.3 .multidot.V.multidot..PSI. (2)
wherein K.sub.1, K.sub.2 and K.sub.3 respectively represent positive coefficients.
However, when the contribution of the third term, K.sub.3 .multidot.V.multidot..PSI., to the target rear wheel turning angle TG.theta..sub.R becomes too large, the effect of the first term, -K.sub.1 .multidot..theta..sub.F, that is, the effect that the rear wheels are momentarily turned in the reverse phase and the heading performance is improved cannot be sufficiently enjoyed.